Known gas turbine power plants include combined cycle gas turbine power plants and single cycle gas turbine power plants. A combined cycle gas turbine power plant includes components such as a gas turbine, a heat recovery steam generator, a steam turbine, and a generator. The gas turbine is employed as prime mover. It can convert the chemical energy of gaseous or liquid fuel into mechanical energy, exhaust enthalpy, and waste heat. The exhaust enthalpy can be used to generate steam, which can subsequently be expanded in the steam turbine. The steam turbine can produce mechanical power. The gas turbine and the steam turbine can be mechanically coupled to a generator. The generator can convert the mechanical power into electrical power. A variety of different combined cycle power plant configurations exist, for example, single shaft or multi shaft configurations. In a single cycle gas turbine power plant, a gas turbine is used in conjunction with a generator only. The step of recovering the gas turbine's exhaust enthalpy is omitted.
In order to enhance the performance of a gas turbine power plant, a number of power augmentation measures have been developed. An example of such a power augmentation measure is the injection of water into the air intake of the gas turbine's compressor.
In power plants, the electrical power output can be regulated by a control system. In the case of a combined cycle power plant, the power output is the sum of the contribution of the gas turbine and the contribution of the steam turbine. At high loads, the steam turbine can be operated in sliding pressure mode. If the steam turbine is operated in sliding pressure mode, the power output of the plant can be determined by the prevailing operating conditions of the gas turbine. The gas turbine can be controlled by a load/temperature controller. The load/temperature controller receives a load set-point (for example, the manual load set-point entered by the operator) and commands the input variables to the gas turbine (for example, the position of the variable inlet guide vanes, the (mixed) turbine inlet temperatures, the turbine outlet temperature(s)) such that the desired power output can result. The manipulated variables can be adjusted within a predefined range and according to a given operation concept. Moreover, the load/temperature controller may consider additional operation limits. These limits can arise from limiting values of selected power plant process quantities (for example, maximum/minimum temperatures or pressures). Maximum power output (also referred to as base load) results when the manipulated variables take their base load values.
The power generated by the plant can be sold to a customer and delivered to the electrical grid for distribution. In order to be able to efficiently dispatch the electricity, it is useful for the operator of the plant to know the maximum power generation capacity. However, in a single cycle or combined cycle gas turbine power plant, the maximum power generation capacity is generally unknown because it is highly dependent on the ambient conditions at the power plant (for example, ambient temperature, ambient pressure, ambient humidity) and other variable factors, such as fuel type and quality (for example, caloric value), heat soaking of the components, aging, dirt.
Among known methods, no simple method (for example, a method of low computational complexity) exists to accurately estimate on a real-time basis the maximum power generation capacity of a single cycle or combined cycle gas turbine power plant.
Knowledge about the maximum power generation capacity is not only valuable for dispatching but can be similarly useful when the power plant operator is asked by the customer (or is contractually obliged by the grid authority) to maintain a specified power capacity in reserve. Such a power reserve is, for example, used in order to provide frequency support. The specification of the power reserve may involve a static requirement (for example, to provide a power capacity of a given MW figure) as well as a transient requirement (for example, to provide the power capacity with a minimum average loading gradient). The specification can be formulated in terms of a desired power capacity and a desired maximum time within which the desired power capacity has to be provided.
Due to internal dynamics, a power plant is not able to immediately follow an increase in demanded power. If the steam turbine is operated in sliding pressure mode, the response of the steam turbine is slower than the response of the gas turbine. These different time constants explicitly have to be accounted for if a desired power reserve is to be maintained. Where the specified loading gradients can be met only by the gas turbine, the controller keeps in reserve a total power capacity that is larger than the power capacity specified.
In the U.S. Pat. No. 6,164,057, a controller is disclosed that operates a gas turbine such that a desired power generation capacity can be maintained in reserve. The inlet guide vane angle of the compressor is applied as an indicator of the reserve capacity of the gas turbine. The proposed controller can continuously compare the actual inlet guide vane angle to an intended inlet guide vane angle that corresponds to a desired reserve capacity. A controller can adjust the fuel flow to the gas turbine to adjust the turbine output power and thereby maintain the actual inlet guide vane angle at the intended value corresponding to the desired reserve capacity.
In the U.S. Pat. No. 6,164,057 the inlet guide vane controller asymptotically compensates the reserve capacity controller's response for frequency support. As a consequence, the method applies exclusively for environments where only short-term frequency support is desired.
Due to the action of the inlet guide vane controller, the power output of the gas turbine generator is independent of the manual load set-point on a long-term basis. As a consequence, the gas turbine generator cannot be operated at a settable load while the reserve capacity controller is active.
The inlet guide vane angle is proposed to be a good indicator of the reserve capacity of the gas turbine. This correlation is assumed to be relatively independent of disturbances, such as changing ambient conditions. However, the bandwidth of the inlet guide vane controller has to be set comparably small in order not to interfere with the control actions of the speed/load governor. As a consequence, only low-frequency disturbances are rejected.